In an image forming apparatus such as a printer, a facsimile, a copy machine, a multi-functional peripheral, or the like, in which an image is formed based on image data and is transferred onto a recording sheet, a latent image is formed on a photoconductor based on image data, and the latent image is developed with a developer, and the developed image is transferred directly, or via an intermediate transfer section, onto a recording sheet. During the transfer, the above-mentioned developed image is transferred onto a recording sheet by pressing a transfer section, which includes a transfer roller, a transfer belt, or the like, into contact with an image bearing member such as the above-mentioned photoconductor or intermediate transfer section, and passing the recording sheet between the part of pressure contact (also referred to as a transfer nip portion).
The above-mentioned transfer section can be driven to rotate by an image bearing member by pressing the transfer section into contact with the image bearing member, which is driven to rotate. However, it becomes difficult for the transfer section to be driven in cases in which a load is given onto the transfer section, and therefore, there are cases in which a transfer section drive section, which drives the transfer section to rotate, becomes necessary. As an example, in cases in which an inter-paper patch correction, or the like, for improving productivity, is carried out, it is necessary to do so while the transfer section is pressed into contact with the image bearing member. Therefore, in this case, a toner attached on the inter-paper patch adheres onto the transfer section. Image forming apparatuses, which are equipped with a cleaning member for transfer section in order to remove the toner adhered onto the image bearing member, have been suggested. The cleaning member, which is a blade, or the like, in general, is pressed into contact with the surface of the transfer section such as a transfer roller, a transfer belt, or the like. Therefore, a load is given onto the transfer section due to the contact of the cleaning member under pressure. In cases of image forming apparatuses equipped with a transfer section cleaning section, because it is difficult for a transfer section to be driven by an image bearing member as described above, a transfer section drive section to drive the transfer section is equipped (for examples, refer to Unexamined Japanese Patent Application No. 2008-304552 and No. 2009-9103).
As just described, in cases in which an image bearing member and a transfer section is to be driven to rotate individually, it is necessary not to deteriorate the accuracy of image forming by the effect of the rotation of transfer section on the rotation of image bearing member. In Unexamined Japanese Patent Application No. 2008-304552, the load onto the image bearing member, due to the rotation of transfer section, is reduced by controlling the driving power, given to the transfer section, in accordance with at least either one of the usage history of cleaning member or the amount of water in the air. In Unexamined Japanese Patent Application No. 2009-9103, by detecting a torque command value of the intermediate transfer section, the control of intermediate transfer section is prevented from being broken down by changing the speed of the transfer section when that command value exceeds a lower limit.
Further, as disclosed in Unexamined Japanese Patent Application No. 2008-304552, the torque given to an image bearing member is kept constant regardless of existence or non-existence of a sheet at a transfer section, by a) providing a torque limiter to the drive system of the transfer section, b) setting the limiter value to “the load of the transfer section (mainly, load from cleaning member)+α”, c) setting the transfer section to rotate a little faster than the image bearing member so that the transfer section presses the image bearing member slightly (namely, torque of “+α”: the value of α is in a range of values with which the total load (the load of the transfer section+α) does not become less than the original load without a even if the original load is reduced by a fluctuation due to periodic speed fluctuation, or the like) in the state in which the transfer section is pressed into contact, and d) activating the torque limiter in that state in which the transfer section presses the image bearing member slightly.
However, as shown in FIG. 6, in cases in which transfer section 30 is driven to rotate by being pressed into contact with image bearing member 31 (in this case, an intermediate transfer belt), if sheet P is passed through the part of pressure contact, the radius of rotation of transfer section 30 is changed by the thickness of sheet P. Therefore, when transfer section 30 is controlled so as to rotate at a constant speed, the torque given to image bearing member 31 varies by the passage cycle of sheet P, and as a result, the speed of image bearing member 31 is changed and color deviation, or the like, occurs, resulting in the problem that the accuracy of image formation deteriorates. In the above-mentioned Unexamined Japanese Patent Application No. 2008-304552, although it is aimed to reduce the fluctuation of torque to the image bearing member, the degree of accuracy is low and it is difficult to solve the fluctuation of torque sufficiently. Also, in Unexamined Japanese Patent Application No. 2009-9103, it is difficult to obtain the derived effect of reducing fluctuation in delivery of torque between the transfer section and image bearing member, thus, the above-mentioned problem is be solved.
Further, in a method in which a torque limiter is used, when the fluctuation (chronological/environmental) of the load (mainly from a cleaning member) of transfer section 30 is large, there is a problem that a limiter value cannot be set. More specifically, in cases in which the load becomes heavier and exceeds the limiter value, the driving at the transfer section side always runs idle, and therefore, cannot drive at a predetermined speed. In cases in which the load becomes too low, the transfer section pushes the image bearing member until the limiter value, resulting in the situation, which is the problem to be solved in Unexamined Japanese Patent Application No. 2009-9103, in which the load onto the image bearing member becomes too low (becoming a minus load in extreme case), and resulting in failure of the control or occurrence of significant color deviation in image forming.